Many camera systems capture archival images using an optical system containing an automatic focus lens with a limited depth of focus. In these systems, the user generally centers the subject in the viewfinder and the camera lens autofocus control algorithm moves the lens focus position based on the sharpness of the subject either at a single spot or in multiple zones. Some cameras alternatively use a distance measure feedback from a discrete sensor to set a focus position. However, since in many cameras the user is not seeing the scene through the archival optical system, it is not possible for the user to determine whether the captured image is focused at the correct depth, and therefore, on the correct subject.
In cameras that incorporate an electronic image capture system, a liquid crystal display (LCD), or other type of electronic display is often used to present an image of the photographic scene. This image can be reviewed by the photographer while the photographer is at the scene of the photograph. This permits the photographer to verify whether the captured image is acceptable and to determine whether the photographer should capture another image of the scene.
Cameras having the capability to present an image of a captured scene to a camera user are referred to generally herein as verifying cameras. Verifying cameras form two images during image capture: an archival image and a verification image. The verification image is an image that is adapted to match the appearance of the archival image. The verification image can be derived from the archival image as is typically done with digital still cameras. The verification image can also be captured using an image capture system that is separate from that used to capture the archival image. This is done in cameras such as hybrid film/electronic cameras as described in combination cameras incorporating dual electronic image capture systems as described in U.S. Pat. No. 5,710,954.
One drawback of the use of verifying cameras is that most displays currently in use with such cameras are relatively small, thus making it difficult to determine the sharpness of the captured image. This is a particular problem when the verifying camera includes a so called “digital zoom” or “pseudo zoom” capability. In these cameras, the captured archival image is cropped and magnified before it is used (e.g. before the film is printed, or before the digital file is stored), so that the full image area of the film or electronic sensor is not used. In such a camera, the sharpness of the captured rendered image (e.g., a print made from the cropped magnified archival image) is greatly reduced as the level of zoom increases. However, this decrease in sharpness may not be readily apparent in the verification image that is presented on the display.
Thus, what is needed is a camera system that presents a verification image that indicates which portions of a captured archival image are out of focus.
Commonly-assigned U.S. Pat. No. 5,103,254, entitled “Camera with Subject Highlighting and Motion Detection,” discloses a camera in which a gradient operation is performed on an electronically captured image, in order to produce an outline of subjects within the depth of field. This outline is displayed using a liquid crystal display (LCD) as a mask to highlight the in-focus subject within the camera's viewfinder. U.S. Pat. No. 5,496,106, entitled “System and Method of Generating a Contrast Overlay as a Focus Assist for an Imaging Device,” discloses a system in which an image is split into its red, green, and blue components, a contrast signal is generated, and the contrast signal is combined with one of the color channels to produce a false-color overlay. Although the methods described in these patents provide focus detection, additional processing of the image is needed to produce the focus detection information. Further, overlays do not indicate out of focus areas in an intuitively understandable manner. An untrained user can misconstrue the meanings of the overlay markings. This in turn can cause the user to make incorrect decisions regarding whether to capture an additional image of the scene. Thus, what is needed is a method and camera that is adapted to present verification images on a display in a manner that intuitively indicates which portions of the archival image are out of focus.